Chronic periodontitis affects half of adults in the US, and pathogenesis results from polymicrobial---host interactions that are not completely understood. Using next generation sequencing technology many new candidate species have been associated with disease, and others with health. But epidemiologic data does not establish causality, so the role of most of these species in periodontitis remains hypothetical. In attempting to understand pathogenesis, a holistic view that includes contributions from both microbes and host is needed. If we are to unravel host interactions with highly complex bacterial communities, we will need to systematically screen in a high throughput system that models host---microbe interplay. And for clinical relevance it is important to ground studies in human epidemiologic data to target candidates from the large number of possible combinations. Deep---sequencing data generated by our group on species prevalence in health and disease, and co---occurrence in individuals, will be used to guide the selection of candidates for the proposed studies. The genetically tractable animal, Drosophila melanogaster, shares many mammalian innate immune response features, has been well developed as a model host---microbe system, and has been adapted for oral microbiology research by our group. The natural infection (feeding) model provides a high--- throughput, powerful approach that allows us to detect and understand in vivo interactions between oral polymicrobial communities and the host. The Drosophila model also allows comprehensive and simultaneous monitoring of bacteria and host gene expression profiles during infection, using RNA---seq. The aims of this project are to identify virulent and beneficial oral species and to identify virulence---enhancing and ---attenuating interactions between oral species, in the Drosophila model. Then candidate bacterial and host factors that are important for these interactions will be identified using RNA-- seq, and tested using Drosophila gene knockout mutants and/or RNAi knock---down lines. This project will be a considerable step forward in understanding the pathogenesis of periodontitis in a holistic view that includes interactions of microbes with one another and the host. Ultimately this work could lead to therapeutic interventions on the host or bacterial side. If beneficial interactions are discovered, translation to chairside as probiotic therapies could occur very rapidly. Strategies to target virulence mechanisms would have a longer horizon, but are highly likely to be suggested by the project.